1. Field of the Invention
The present invention relates to: an induction heating device serving as a source of heat, for example of a fuser of an image forming apparatus; and an image forming apparatus with this induction heating device being installed thereon.
2. Description of the Related Art
The following description sets forth the inventor's knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art.
Some of the image forming apparatuses such as copiers, printers, and facsimiles and the multifunctional digital image forming apparatuses called as MFPs (Multi Function Peripherals), collectively having the functions of these image forming apparatuses, use an induction heating device as a source of heat for heating the fuser rollers of fusers.
Such an induction heating device, which has a resonant circuit with an induction heating coil and a capacitor being coupled in parallel with each other and a switching element constituting an IGBT (Insulated Gate Bipolar Transistor) which is coupled in series with the resonant circuit, conventionally has been employed a method of achieving control of power to the fuser rollers: performing full-wave rectification to convert a commercial AC voltage to DC; applying it to the resonant circuit; and controlling an ON period of the switching element.
Such an induction heating device having a resonant circuit and a switching element as described above makes the switching element turn power ON when the collector-emitter voltage of the switching element becomes approximately equal to the input voltage of the resonant circuit, after going up and down. Turning power ON at that time, the switching element does not have to cause much switching loss.
However, when the power to the resonant circuit is reduced by power control, the switching element cannot obtain an enough amount of current because the collector-emitter voltage decreases with reduction of the power; and while the collector-emitter voltage hardly can fall to the same level as the input voltage, the switching element has to be turned ON and OFF. As a result, the switching element causes too much switching loss and sometimes can be broken for that reason, which is why it has been difficult to achieve control of power to the fuser rollers when reducing the power.
To solve such a problem, Japanese Unexamined Patent Application No. 2002-328553 discloses a technology to reduce a switching loss of the switching element by changing the capacitance of the resonant capacitor.
And Japanese Unexamined Patent Application No. 2009-295392 discloses a technology to prevent any fusing failures by controlling the ON/OFF state of the switching element in synchronization with a zero-crossing of the AC voltage and changing the time of the control pulse of the switching element every half cycle.
However, there is a problem with the technology disclosed in Japanese Unexamined Patent Publication No. 2002-328553 that a configuration to change the capacitance of a resonant capacitor is complicated and costly.
And there is a problem with the technology disclosed in Japanese Unexamined Patent Publication No. 2009-295392 that a commercial voltage sometimes can drop to cause flickering in a lighting apparatus such as a fluorescent lamp, because of too much power to a workpiece to be inductively heated.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. Indeed, certain features of the invention may be capable of overcoming certain disadvantages, while still retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.